Characterization of plant cation/h+ antiporters and how they can impact nutrition
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Calcium transporters regulate calcium fluxes within cells. Plants, like all organisms, contain channels, pumps and exchangers to carefully modulate intracellular calcium levels. Numerous Arabidopsis proteins have been characterized which can transport calcium. However, there are numerous genes whose products have not been fully characterized. One method I used to infer function was to analyze various promoter lengths of 17 putative cation transporters fused to reporter and to observe changes in the reporter expression in response to various stimuli. Using a more in depth approach I set out to characterize the function of AtCCX3. Here I show Arabidopsis AtCCX3 can suppress yeast mutants defective in vacuolar Na+ and K+ transport. AtCCX3-expressing yeast cells conferred Mn2+ sensitivity when highly expressed. Functional epitope tagged AtCCX3 fusion proteins were localized to endomembranes in plants and yeast. Expression of AtCCX3 increased in plants treated with NaCl, KCl and MnCl2 and caused increased Na+ accumulation and increased K+ transport. Ectopic expression of AtCCX3 in tobacco produced lesions in the leaves, stunted growth, resulted in the accumulation of higher levels of numerous cations and increased protein oxidation preceded alterations in leaf growth. These characteristics define AtCCX3 as an endomembrane localized transporter with biochemical properties distinct from previously characterized plant transporters. In the remaining studies I used the mouse model to determine how molecular changes to plants can improve the nutritional quality of the food. Previously, the cod5 M. truncatula mutant was identified which contains identical calcium concentrations to wild-type, but contains no oxalate crystals. Mice fed intrinsically labeled cod5 plants had 22.87% (p<0.001) calcium absorption compared to wild type plants. In a second study, using mice I examined if increased expression of a calcium transporter which increases calcium concentration 2-fold, alters bioavailable calcium. In mice feeding regimes (n = 120), I measured 45Ca-incorporation into bones, and determined that mice required twice the serving size of control carrots to obtain the calcium found in sCAX1-carrots. Together, these two studies demonstrate how the potential utility of removing calcium absorption inhibitors and fortifying vegetables can improve calcium bioavailability.
Morris, James Larry (2007). Characterization of plant cation/h+ antiporters and how they can impact nutrition. Doctoral dissertation, Texas A&M University. Available electronically from